This invention relates to a heat exchanger and, more particularly, a shell and tube heat exchanger or tubular heat exchanger for effecting a heat exchanging operation between low temperature gas and high temperature gas.
There is known a gas-gas heat exchanger, for example, a shell and tube heat exchanger, in which a low temperature gas (about 100.degree. C.) to be fed into a dehydrogenation reactor in a styrene monomer manufacturing apparatus is heated and, simultaneously, a high temperature gas (about 500.degree. to 600.degree. C.) from the dehydrogenation reactor is cooled. It is generally required for the shell and tube heat exchanger of this type to be arranged in series of a heat exchanger located downstream thereof on the tube side because of the arrangement of equipment required for the process. In addition, with the heat exchanger of this type, it is also necessary to absorb the expansion difference between thermal expansions of the heat exchanger .tube (hereinafter called the "tube") and the shell due to the average metal temperature difference between the tube side and the shell side. In order to solve these problems, a conventional heat exchanger is generally constructed so that both the tube side and the shell side have one pass and a rear tubesheet is formed into a floating type or the conventional heat exchanger is constructed as a fixed tubesheet heat exchanger in which an expansion joint is arranged outside or inside the shell.
With the conventional heat exchangers of the types described above, a front tubesheet is composed of a welded attachment structure (Type D) as shown in FIG. 11 in which tube and tubesheet are welded and a rear tubesheet is composed of a welded attachment structure (Type C) as shown in FIG. 10 in which the tube and the tubesheet are welded and a metal wall of the tube is expanded towards the tubesheet. Namely, the Type D is a structure in which the end portion of the tube is inserted into an end opening of a tube hole in the tubesheet up to the end of larger diameter portion of the tube hole and then welded. The Type C is a structure in which the tube is inserted into the end opening of the tube hole in the tubesheet so that the front end of the tube protrudes from the tubesheet, the front end of the tube is welded to the tubesheet and a tubular portion of the tube inserted into the tubesheet is expanded before or after the welding operation.
The Japanese Patent Laid-open Publication No. 50-76638 discloses a heat exchanger in which a frustoconical surface is formed so that a point of a virtual cone is positioned in a tube hole of a tubesheet and in which groups of tubes inserted into the tubesheets are welded at the tube end sides.
The conventional heat exchanger assembled in the styrene monomer manufacturing apparatus of the type described above tends to cause problems in that, since the heat exchanger is generally operated under a pressure as low as possible near the atmospheric pressure, the maximum allowable pressure loss for the actual operation is extremely small. Hence, it is difficult to keep sufficiently short the distance between each two adjacent baffles on the side of the shell, resulting in the generation of flow-induced vibration of the tube. Moreover, in such a heat exchanger, the gas flow is liable to stagnate at a portion near the tubesheet on the shell side, at which paint carbon contained in the gas is liable to precipitate as carbon particles.
With the Type D structure described above, it is impossible to substantially completely eliminate the gap between the tube and the tubesheet and a small gap, even an extremely small gap, remains. For this reason, when the carbon is precipitated, the carbon particle intrudes into the gap and the carbon particle gradually grows in the gap as time elapses for a long time operation into a massive solidified carbon particle, which may press inwardly and finally deform the tube (which is a so called necking phenomenon for the tube). In an adverse case, such deformation will damage the welded portion between the tube and the tubesheet or break the tube, thereby resulting in leakage of the gas from the shell side to the tube side.
Such an adverse phenomenon is more liable to happen on the side of the front tubesheet, but may be observed on the side of the rear tubesheet. This problem is also significant, as is the problem of the flow-induced vibration of the tube described above for the conventional heat exchanger.
In addition, the tube tubesheet weld attachment structure disclosed in the Japanese Patent Laid-open Publication No. 50-76638 has been proposed for the purpose of preventing the stagnation of liquid and, for this purpose, an obtuse-angled taper is formed at the tube hole in the tubesheet. This imparts restrictions in the formation of the tubesheet, design for improving the strength, thickness of the tubesheet, pitch of the tube arrangement, etc. Accordingly, the structure of this prior art lacks wide utilization.
SUMMARY OF THE INVENTION
An object of this invention is to substantially eliminate the defects or drawbacks encountered in the prior art described above and to provide a heat exchanger provided with an improved tube - tubesheet welded attachment structure capable of substantially completely eliminating gaps between the tubes and the front and rear tubesheets and of preventing the tube and the welded portion between the tube and the tubesheets from being damaged, even in a case where carbon is precipitated near the tubesheets during the operation of the heat exchanger.
Another object of this invention is to provide a heat exchanger provided with baffles having a structure selected suitably from some applicable baffle types superior in the flow-induced vibration preventing characteristics of the tubes.
These and other objects can be achieved according to this invention by providing a heat exchanger of the type having a rear tubesheet of a floating structure type or a stationary tubesheet in which an expansion joint is disposed inside or outside of a shell of a heat exchanger to absorb an expansion difference, due to thermal expansion of a tube and the shell. The heat exchanger comprises a shell forming a body of a heat exchanger, tubesheets comprising front and rear tubesheets disposed inside the shell at portions near longitudinal ends of the shell, a plurality of tubes extending between the front and rear tubesheets, and baffles arranged inside the shell along a longitudinal direction of the heat exchanger tubes. Each of the tubes has an outer diameter in the range of 25.4 to 50.8 mm. The front tubesheet has a thickness less than 50 mm, and the tubes and the front and rear tubesheets are welded to substantially eliminate gaps therebetween. The heat exchanger of the present invention is utilized in a combination of the following welded attachment structures of: (a) a structure (Type A) in which the tubesheet is provided with a hole having an inner diameter substantially equal to that of the tube and with a protruded peripheral portion formed around an inside opening of the hole and having an outer diameter slightly larger than an outer diameter of the tube, the protruded peripheral portion having an inside stepped cutout portion in a circumferential direction thereof to form a stopper portion into which one end of the tube is inserted in abutment thereto and welding is carried out between the inserted end of the tube and the tubesheet; and (b) a structure (Type B) in which the tubesheet is provided with an inner tapered hole and a linear small diameter hole communicating at one end with a small diameter portion of the tapered hole, a tube having an end portion provided with an inside stepped cutout in a circumferential direction thereof being inserted into the small diameter hole, and welding being carried out between the inserted end of the tube and a portion of the tubesheet surrounding the small diameter hole.
In preferred embodiments, the tube and the tubesheets may be welded by adopting the combination of the welding structures of the Types A and B in the following manner.
The rear tubesheet has a thickness more than 50 mm, the tube bundle is provided with "Segmental No Tube In Window Type" baffles, or "Segmental Type" baffles, or "Double Segmental Type" baffles, the front tubesheet is welded to the tube in the form of the structure of Type B, and the rear tubesheet is welded to the tube in the form of the structure of Type A.
The three types of baffles mentioned above are defined as follows.
Segmental No Tube In Window Type (hereinafter called "NTIW Type"): each of the baffles is formed in a circular shape having a cutout portion, and the tubes are arranged only at a portion at which parts of adjacent baffles are overlapped;
Segmental Type: each of the baffles is formed in a circular shape having a cutout portion, and the tubes are arranged fully in the shell.
Double Segmental Type: each of the adjacent baffles are arranged in a combination of a circular baffle having plural cutout portions at opposite ends and the other circular baffle having a cutout portion at its center, and the tubes are arranged fully in the shell.
The rear tubesheet has a thickness less than 50 mm, the tube bundle is provided with Segmental Type baffles, or Double Segmental Type baffles, the front tubesheet is welded to the tube in the form of the structure of Type A and the rear tubesheet is welded to the heat exchanger in the form of the structure of Type B.
The rear tubesheet has a thickness less than 50 mm, the tube bundle is provided with Segmental Type baffles, or Double Segmental Type baffles, the front tubesheet is welded to the tube in the form of the structure of Type B and the rear tubesheet is welded to the tube in the form of the structure of Type A or B.
According to the invention having the characteristics described above, the tubes and the front and rear tubesheets of the heat exchanger can be welded in a suitable welding mode to substantially completely eliminate gaps between the tubesheets and the tubes. The welding with the Type A structure may be classified essentially as butt-welding of tubesheet and a tube. The welding is carried out from the inside of the tube and the completely fused penetration will be achieved at the welded portion. The welded condition has to be inspected from the outside of the tube to confirm the quality of the welded portion. The welding to the Type B structure is suitable for the welding of the tubesheet having a relatively thin thickness, less than 50 mm to the tube having an outer diameter of 25.4 to 50.8 mm, for example. The welding operation is carried out from the front side of the tubesheet with the abutting condition of the tube and the tubesheet. The welded condition will be confirmed from the front side, i.e. welding side, of the tubesheet.
Accordingly, the gaps between the front and rear tubesheets and the tubes can be substantially completely eliminated by adopting the suitable structures thereof to avoid the necking phenomenon of the tube even if carbon is precipitated near the tubesheets during the operation of the heat exchanger. The type of the baffles may be also selected suitably in accordance with the flow-induced vibration analysis of the tubes.